A major biological influence on community structure is the presence of algal canopies and shorter algal communities at mid and low shore levels. Macroalgae provide a variety of resources which are not available on bare rock. Most importantly, they increase the amount of space available for attachment, they provide shelter from wave action, desiccation and heat and they are an important food source.

The presence of algae can have positive effects on some species and negative effects on others, as illustrated by the interaction between Fucus vesiculosus, limpets and barnacles. Algal canopies reduce light availability to understorey plants and can trap silt which smothers some organisms at the same time as providing an environment for burrowing species. Removal of high shore canopy forming species often leads to a proliferation of turf forming algae including some species which are usually more common as epiphytes (Hawkins and Harkin, 1985). Higher on the shore, such canopy species are damaged when Ascophyllum spp. are removed (Hawkins, 1979; Jenkins, 1995)

Bryozoans, hydroids and Spirorbis spp. grow on algal fronds at low shore levels. A more mobile fauna colonises higher shore algae. Epiphytic algae are also common on kelps and fucoids. These provide food for grazers and predators while the brown macroalgae also provide food for grazers such as Littorina obtusata and blue rayed limpets. Sediment stabilised by the canopy is further stabilised by the sand-binding red alga Audouinella floridula and colonised by infaunal species. The net result of algal canopies and turfs is usually an increase in species richness (see Williams and Seed, 1992) for review.

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